Of the various types of valves utilized for controlling the flow of fluid within conduit systems, rotary plug valves have been found to be quite popular. Rotary plug valves are typically of compact design when considered in comparison with many other types of valves having substantially the same flow passage dimension where physical size and height is an important or mandatory design consideration. Rotary plug valves such as tapered plug valves and ball valves are widely used. Rotary plug valves are typically of simple nature, utilizing few moving parts and are of nominal cost, and these features substantially enhance the competitive nature of plug valves.
Among the disadvantages of rotary plug valves, however, are the disadvantages of operational pressure range and sealing capability. Most rotary plug valves function properly only at relatively low pressure ranges, for example in the order of 0-150 psig. This operational pressure restriction, in many flow systems, requires that other, less desirable, more expensive but higher pressure valves be utilized. Where plug valves incorporate lubricant enhanced sealant, additional sealing must be introduced into the sealant chambers quite often to insure against leakage. This requirement is an expensive service consideration which is frequently detrimental to the employment of such valves. Also, the lubricant sealant must be compatible with the product controlled by the valve because a small amount of the sealant becomes lost into the flow stream.
In may cases, elastomeric sealing material is incorporated in the plug valve mechanism for establishment of a seal between the rotatable plug and the sealing surfaces of the valve body. The elastomeric sealing material tends to wear at a rapid rate, especially in valves of larger size, and therefore presents a problem from the standpoint of servicing, especially if the valve is intended for frequent cycling during use.
To gain the advantages of reasonably high operational pressure and extended service life of sealing material, plug valves have been developed that incorporate mechanically enhanced sealing. In one type of valve mechanism for this purpose, (known as a "lift-turn" plug valve) a rotatable plug element having a flow port formed therein is formed to define external interlocking connections and tapered cam surfaces that mate with interlocking connections and cam surfaces defined by a pair of opposed slip elements. The slips incorporate sealing elements, typically of the molded elastomeric or resilient type, and in the closed position of the plug member, establish sealing engagement with sealing surfaces defined within the valve body as the plug member is moved linearly within the valve body. The cooperating cam surfaces of the tapered plug member and the slips are operative to induce lateral movement of the slips to and from sealing engagement within the valve body. A valve actuator typically parts vertical movement to a valve stem and a cam follower of the actuator typically transverses an L-shaped groove to control linear and rotational movement of the valve stem and plug member. U.S. Pat. No. 3,793,893 of Heinen and 3,046,802 of Cupedo are representative of lift-turn valve actuators of this type. U.S. Pat. No. 4,234,157 of Hodgeman, et al., commonly assigned herewith is also generally representative of a lift-turn valve actuator for tapered plug valves having slip assemblies for high pressure sealing capability. In these types of high pressure rotary plug valves, seal life is materially enhanced due to lateral movement of the slips into direct sealing engagement with the seating surfaces within the valve body and the camming activity that occurs between the plug and slips as the plug is moved linearly while the slips are restrained from linear movement. High pressure service capability is therefore promoted by cam induced mechanically enhanced sealing.
In rotary plug valves with slip sealing assemblies, in the open position of the valve it is desirable that the sealing slips with their resilient seals be disposed in spaced, protected relation with the inner surfaces of the valve body and also protected from the flow stream. During rotation of the plug and slip assembly for opening and closing of the valve, it is appropriate that the sealing surfaces of the slips be maintained in spaced relation with the internal sealing surfaces of the valve body so that seal erosion will not occur during plug rotation. After the plug member has been rotated to its closed position with its flow port disposed in transverse relation to the flow passages of the valve, the sealing surfaces of the slips are then moved laterally into direct sealing engagement with the internal sealing surface of the valve body about the flow passages without any rotation of the plug member. This feature permits the establishment of effective sealing capability even under high pressure conditions without inducing any mechanical erosion of the seal members while closure and sealing of the valve mechanism is being accomplished. The reverse is true upon opening of the valve mechanism. The plug member is moved vertically by the valve actuator thereby inducing the interacting cam surfaces of the plug member and slips to retract the slips directly from sealing engagement with the sealing surfaces within the valve body prior to rotation of the plug and slip assembly to its open position. As the valve is being opened initial upward movement of the plug member induces retraction of the slips. The upstream slip will be forced inwardly toward the plug member and away from its sealing relation with the valve body by upstream pressure and flow of fluid. The downstream slip, however, will maintain its sealed relationship with the sealing surface of the valve body by the force that is developed by the pressure differential that exists across the downstream slip. Further vertical movement of the tapered plug member together with restraint of vertical slip movement by the valve body structure, induces inward, retracting movement of the downstream slip until its seal with the body surfaces is broken, thereby permitting limited flow to the downstream flow passage but materially decreasing the pressure differential across the downstream slip. With both of the slips retracted positively clear from the internal sealing surfaces of the body, the plug member and slips may then be rotated in unitary fashion to the open position, thereby bringing the flow port of the tapered plug into proper registry with the flow passages of the valve body.
In the closed position of a high pressure plug valve of this type, double block and bleed sealing can be accomplished by venting the valve chamber. When this is done, any leakage across either the upstream or downstream seals of the slips will be capable of detection.
As mentioned above, many valve actuators for lift-turn type plug valves accomplish rotation of the plug member between its open and closed positions in response to linear movement of the valve stem by a stem actuator that imparts linear movement to the stem. I is desirable, therefore, to provide a valve actuator for lift-turn type plug valves, especially those with cam energized slip assemblies for high pressure sealing capability, wherein linear movement of a valve stem is accomplished in response to rotary movement of a valve actuator component and the resulting valve and actuator assembly will be of limited height.